Autoregulation of cerebral blood flow has been demonstrated in both fetal and newborn animal models under normoxic conditions. In the present experiments we have attempted to define the minimal hypoxic insult which impairs autoregulation in the newborn lamb and to assess the time to recovery. We measured cerebral blood flow by the intracarotid 133Xe method in fifteen 4- to 9-day-old lambs and tested autoregulation of cerebral blood flow by increasing blood pressure 20-30% with phenylephrine. Autoregulation was tested in the control state and at successive time intervals after an hypoxic stress (PaO2 of 30 mm Hg) of 10 or 20 min. We found that cerebral autoregulation was abolished after 20 min of hypoxia and recovered within 7 h. Since this model identifies the minimal hypoxic insult to abolish autoregulation it might be used to study means to protect autoregulation or to hasten its recovery after hypoxia.